The interest for biological methods for inhibiting pest insects has steadily increased during the last decade. This is particularly due for pheromones, the specie specific signal substances which some animals, in particular insects, communicate with, inter alia to find a partner prior to mating. Pheromone based inhibiting methods against pest insects are characterized in that 1) they are highly species specific, 2) the chemical compounds used are relatively simple, and biologically active in very small amounts to the target organism, often nanogram amounts, and 3) the insects have great difficulty in developing resistance against these natural signal compounds.
The high species specificity of the pheromones means that pheromone based inhibiting agents are directed almost only against individuals of one single species. Sometimes, furthermore, against closely related species. The chemical composition of the pheromones is as a rule of such a type that they will quite easily decompose by UV-light and microorganisms after an application. Then they are decomposed to compounds which are completely harmless to other organisms. This is natural as the pheromones as a behavior signal shall be short lived. They are thus not accumulated in higher organisms in a nutritional chain. The development of resistance in the insects against species specific pheromones is extremely improbable. In spite of a considerable use of, pheromones in the inhibition of e.g., a cotton fly in California for a whole decade, no resistance has developed. No change whatsoever in the composition of the pheromone has been demonstrated.
For the reason given above pheromone based inhibiting methods against pest insects are particularly harmless to the environment. Furthermore, they have proven to be extremely efficient.
Inhibition of forest damaging insects is an area where pheromone based inhibiting methods already are of great importance and have great economic value is at stake. This is particularly true for bark beetles which every year causes great losses both in the form of killed trees, growth losses and other damage, e.g., blueing fungus.
The six toothed spruce bark beetle, Pityogenes chacographus is one of the most serious forest pest insects in Europe. It can under certain circumstances attack and kill standing healthy spruces to a great extent.
The specie normally mates in healthy as well as in dying trees, e.g., wind fallen trees. The larvae live off the nutritionally rich bast layer (phloem) underneath the outer bark. A male that bores into that excretes a pheromone consisting of 2-ethyl-1,6-dioxaspiro-.vertline.4,4.vertline.-nonane, having the trivial name chalcogran (CH) and methyl-2,4-decadienoate (MD). Both substances are used commercially today as attractants. In the case of chalcogran, it is referred to in U.S. Pat. No. 4,205,084, and in the case of methyl decadienoate it is referred to in the Swedish Patent Application SE-A-8600448-8. Of chalcogran the 2S,5R-enantiomer is preferred, and of methyl ester the 2E,4Z isomer is preferred. The pheromone attracts females to the tree but also other males. The pheromone is well known and is utilized since it has long been used on a commercial scale as an attraction agent in traps for catching the six toothed spruce bark beetle.
The amount of pheromone excreted by an inbored male increases during the first days and thereby further males and females of the specie are attracted. By attracting further males he will obtain help in breaking down the defense system of the tree present in the form of a rich resin gum flow. The male will then gnaw out a mating chamber and is ready to allow one or more females therein, which after mating will start laying eggs therein. The amount of pheromone attraction agent then deceases and is down to a very low level about one week after the male has started his boring into the bark. This means that when many spruce bark beetles attack a tree then the attracting pheromone signal from the tree will increase rapidly. More bark beetles will be attracted and the number increases exponentially. Such a massive attack on a certain tree is characteristic for the reproduction strategy of the bark beetles. Certainly, it is, however, devastating if too many bark beetles are attracted to the same tree. The competition for the limited feed for the larvae will then become so great that many of them die. Thus the bark beetles excrete a stop signal for further attraction to that tree.
Usually, the six toothed spruce bark beetle lives in low population densities. In this endemic phase it mates in wind fallen, broken or otherwise damaged trees, which show a low vitality. Severe autumn storms which result in many wing fallen spruce offers, however, an excess of mating trees to the spruce bark beetle. The population thereof increases rapidly, and in particular if two such years follow on each other the spruce bark beetle can reach an epidemic phase in which the population density is so high that it, with success, can attack and kill also healthy spruces. Cut, unbarked spruce, which is left in the forest during the swarming of the spruce bark beetle in May to June is, from the insects point of view, a perfect mating place. In order to prevent the build up of epidemic population densities then piles of cut spruce have to be protected in some way. Today this is done in Sweden through legislation requiring cut spruce to be transported out of the forest before August 1, i.e., prior to the hatching of the new generation of spruce bark beetles.